"We’ve begun testing reusability with something called the Grasshopper Project, which is a Falcon 9 first stage with landing gear that can take off and land vertically," Musk tells Wired.

"The stages go to orbit, then the first stage turns around, restarts the engines, boosts back to the launch site, reorients, deploys landing gear, and lands vertically."

Why is reusability important?

Musk says that the eventual goal is to make humans multi-planetary, and that it won't happen until someone comes up with a rocket that you can use over and over.

It's simply too expensive to make a new craft for every flight. Even the Space Shuttle wasn't truly reusable — the main tank would be thrown away upon return and it would cost a ton to refurbish the parts that were used again.

"The fundamental breakthrough that needs to occur in rocketry is a rapidly and completely reusable rocket. In the absence of that, space transportation will remain two orders of magnitude more expensive than it should be," Musk tells Wired.